Cell lysis is the destruction, or disruption, of a cell's membrane or wall, which breaks open the cell and exposes its contents. Many techniques are available for the disruption of cells, including physical and detergent-based methods. Physical lysis often requires expensive, cumbersome equipment and involves protocols that are difficult to repeat due to variability in the apparatus. Detergent-based methods are often easier-to-use with more efficient protocols than physical methods.
Sonication is one method of physically lysing cells. Sonication uses pulsed, high frequency sound waves to agitate and lyse cells, bacteria, spores, and finely diced tissue. The sound waves are delivered using an apparatus with a vibrating probe that is immersed in the liquid cell suspension. Mechanical energy from the probe initiates the formation of microscopic vapor bubbles that form momentarily and implode, causing shock waves to radiate through the sample. The sonic energy delivered to a sample using this method is variable, and not repeatable for a necessary level of precision.
Cells can be treated with various agents to aid in the disruption process. For example, lysis can be promoted by suspending cells in a hypotonic buffer, which cause them to swell and burst more readily under physical shearing. Alternatively, processing can be expedited by treating cells with glass beads in order to facilitate the crushing of the cell walls.
The resistance of cells and viruses to lysis or disruption is based on the characteristics of the cell membrane, cell wall, or coat protein. The various chemical, enzymatic, and mechanical or physical approaches have been utilized to non-specifically lyse cells and viruses. However, in some applications, it is desirable to lyse one specific cell type or virus in a mixed sample of two or more cells and viruses. One such application is DNA typing.
DNA typing has been an invaluable tool for forensic science. Applications including linking a suspect to a crime site or a victim, identifying a perpetrator via a “cold hit” in a networked crime laboratory DNS database, identifying a victim or human remains, and proving the innocence of wrongly incarcerated prisoners by analyzing archived evidence. Sample types and matrices can vary considerably, and the entire sample preparation process can be very time consuming and labor intensive.
Rape kits, containing swab samples of biological evidence collected in hospitals from victims of sexual assault, are amongst the most common, yet difficult, sample types to process since the swabs potentially contain a mixture of female epithelial cells and male sperm cells. Differential extraction is applied to separate the two distinct cell types into male and female cell lysate fractions, and extract and purify the DNA from each fraction. A resultant genetic profile of the male DNA is compared to that of a suspect, if available, or screened through the crime laboratory DNA database.
The conventional method for separating epithelial cells from sperm cells involves selective lysis using a combination of enzymes, chemicals, heat, and centrifugation. In a mixed sample, the epithelial cells are lysed first due to their lack of a protective coat, the sperm cells are pelleted using centrifugation, the epithelial cell lysate is removed, the sperm cells are re-suspended, and the sperm cells are lysed using more stringent enzymatic, chemical, and heat conditions. This conventional process takes hours, sometimes days. New cost-effective and efficient methods and instrumentation need to be developed and validated for practical low-cost, low processing time, high-throughput solutions.